Enteric anendocrinosis is a congenital diarrheal disorder caused by the loss of the intestinal endocrine cells. Aside from supportive care, this disorder and other related diarrheal disorders with enteroendocrine cell dysgenesis do not have therapeutic options. This proposal focuses on a mouse model of inducible intestinal Arx deficiency (ArxIKO) to understand the mechanism of malabsorption in these disorders. Intestinal Arx deficiency causes impaired enteroendocrine development with diarrhea, lipid malabsorption, and impaired fecal short chain fatty acids. Preliminary data suggests that a low fat diet restores normal growth in the intestinal Arx-deficient mice, implicating a low fat dietary intervention as a therapeutic option. The specific aims outlined will characterize the lineage allocation of enteroendocrine cells in ArxIKO mice and growth on low-fat (10% calories from fat) and high-fat (45% calories from fat) diet. During challenge with a high-fat diet, I will carefully investigate lpid processing in the Arxint mice with both mass spectrometry and colorimetric assays. Ultimately, pancreatic enzyme supplementation will be provided to understand the contribution of secondary pancreatic insufficiency. As a new aim, I will characterize the intestinal microbiota in response to intestinal Arx ablation on the low-fat and high-fat diet, determine the fecal short chain fatty acid content, and perform fecal transplantation to determine the contribution of the microbiota to the pathogenesis of disease. Direct downstream targets will be identified with ChIP-seq, validated in luciferase assays, and overexpressed in intestinal organoid culture in order to rescue enteroendocrine lineages. Finally, a lipid absorption assay will be developed in the intestinal organoids to validate secondary targets. My proposal outlines a 3-year training program for my continued development as a physician-scientist in Pediatric Gastroenterology. As a clinician, I have developed an expertise in congenital diarrhea and am part of our Intestinal Rehabilitation Program. As a scientist, I have an extraordinary mentor in Dr. Klaus Kaestner who has been very generous with his time and resources. The Kaestner lab is also a vibrant community for scientific development. To further promote my career development, I have a Scientific Advisory Committee with whom I will meet twice a year. All members are highly regarded physician-scientists and include Dr. Gary Wu (adult gastroenterologist) and Dr. Doris Stoffers (adult endocrinologist) at the University of Pennsylvania and Dr. Eric Marsh (pediatric neurologist) and Dr. Robert Heuckeroth (pediatric gastroenterologist) at The Children's Hospital of Philadelphia. Additionally, training with Dr. Daniel Rader in the field of lipid biology will exand my education on lipid malabsorption. The combination of the research community and state-of-the-art facilities at The Children's Hospital of Philadelphia and the Perelman School of Medicine at the University of Pennsylvania create an ideal environment for my development as a physician-scientist as I continue to make contributions to the field of pediatric gastroenterology and progress toward an R01 submission.